The use of registration or fiducial marks in semiconductor processing is well known. Registration marks are used to align one pattern layer of metal, insulator, or semiconductor material on a substrate with another pattern layer to ensure that features of the successive layers bear the correct spatial relationship to one another. The features of the registration marks are typically used to align the substrate with the lithographic writing tool being used, such as optical or direct electron beam writing lithography. During the lithography process, the registration mark is observed and used to properly align the lithographic pattern with the underlying layer. In optical lithography the registration mark is typically observed with an optical scanner. Although this method may be used with direct electron beam writing lithography as well, where the registration mark is under a layer of resist, the registration mark is conventionally "observed" by detecting the back scattered electron signal generated when the electron beam contacts the registration mark.
A conventional electron beam used in direct writing lithography typically has a high energy level, in excess of 10 keV and up to 50-100 keV. A high energy electron beam can penetrate a layer of resist with a thickness of approximately 2000 .ANG. to 2 .mu.m and contact an underlying registration mark. As the electron beam penetrates the resist layer, back scattered electrons are produced. By detecting the contrast in the back scattered electron signal caused when the electron beam contacts the registration mark under the layer of resist, the location of the registration mark may be determined. The electron beam is then aligned accordingly.
FIG. 1 is a side view of a semiconductor substrate 10 with a conventional registration mark 12 covered by a layer of resist 14. A conventional high energy electron beam 16 is shown penetrating resist 14 and contacting registration mark 12. Back scattered electrons, which are illustrated as arrows 18, are detected by electron detector 20. As electron beam 16 is scanned across resist layer 14, as illustrated by arrow 22, the contrast in the back scattered electron beam signal detected by detector 20 will indicate the location of registration mark 12. A conventional registration mark is typically a conductor of a material different from the substrate or a physical step or void in the substrate.
Thus, to detect underlying registration marks, conventional electron beams must operate at an energy level that is sufficient to penetrate the layer of resist. Where an electron beam does not have sufficient energy to penetrate the layer of resist to contact the registration mark there will be no contrast in the back scattered electrons to indicate the location of the registration mark.
An example of a electron beam that may not have sufficient energy to penetrate a layer of resist is a miniature electron-beam microcolumn ("microcolumn"). Microcolumns produce low energy electron beams, currently 1-2 keV, and thus may have difficulty detecting registration marks underlying the layer of resist that is greater than approximately 1000 .ANG.. Microcolumns are based on microfabricated electron "optical" components and field emission sources and may be used for direct writing lithography. Microcolumns are discussed in general in the publication "Electron-Beam Microcolumns for Lithography and Related Applications, " by T. H. P. Chang et al., Journal of Vacuum Science Technology Bulletin 14(6), pp. 3774-81, November/December 1996, which is incorporated herein by reference.
Thus, to detect a conventional registration mark, the electron beams must operate at an energy level that is sufficient to penetrate the layer of resist. Where the resist layer has a thickness greater than the penetration depth of the electron beam, one method to permit the electron beam to contact the registration mark and generate a contrasting back scattered electron signal is to remove the resist in the area directly over the registration mark. However, this extra processing step is undesirable because it is complex and time consuming.